Many current synthetic liquid-repellent surfaces are based on the lotus effect: water droplets are supported by microscopic surface textures on a composite solid-air interface that enables them to roll off easily. The microscopic roughness is combined with low surface energy to attain extreme non-wetting properties. Liquid droplets are supported atop texture features with air pockets trapped within the texture. As long as the air pockets are stable, the surface continues to exhibit non-wetting behavior. Such surfaces are commonly classified as superhydrophobic, oleophobic, or omniphobic.
The lotus effect, however, does have some significant challenges. For instance, air pockets within the surface texture can be collapsed by external wetting pressures, can diffuse away into the surrounding liquid, and/or can lose robustness upon damage to the texture. Under condensation, such surfaces display poor drop mobility due to nucleation of droplets within texture features that pin to the surface. The high surface area of microtextures creates more nucleation sites to condense droplets and can induce ice nucleation at an even faster rate than smooth surfaces of the equivalent materials at high humidity conditions. Frost and ice that builds up within the textured features of such surfaces makes ice adhesion significantly stronger than that of smooth surfaces and substantially increasing the amount of energy required to remove the accumulated ice.
Slippery, non-wetting surfaces have been prepared by infusing a lubricating liquid within a microstructured substrate to produce a thin, ultrasmooth lubricating layer that can repel most immiscible materials. Micro-texturing can be introduced by a variety of methods including lithographic patterning of silicon microposts and micromolding of epoxy-based nanostructures from a silicon master. In these cases, the textured material is treated with a low surface energy silane to render the surface compatible with the low surface energy lubricating liquid. This is typically accomplished by placing the substrate in a desiccator under vacuum for at least a few hours. The textured and treated surface is subsequently infused with the lubricating oil applying it in several droplets. These fabrication approaches, however, are multiple step processes and are time and labor intensive. Further, lithographic patterning and micromolding are challenging to carry-out on non-planar surfaces and are not well-suited for some substrate materials, including metals
Alternative approaches include electrodeposition of a conductive polymer on aluminum. The resulting polymer has a roughened texture that is capable of trapping the lubricating liquid. This approach, however, requires surface functionalization with a low surface energy silane to achieve chemical affinity with the lubricating oil. A major drawback of this approach is that it is limited to conductive substrates.
Another approach involves the use a textured membrane, such as a porous Teflon-based membrane. In this case, the Teflon-based membrane material already exhibits a chemical affinity with the lubricating oil and the silanization step is unnecessary. A product or structure requiring non-wetting properties, however, must have the membrane affixed to it. A significant drawback of this approach is the challenge in applying the membrane to non-planar surfaces with complex geometries.
There is a need for robust non-wetting surfaces that afford one or more of the following features: repellency to a variety of liquids, enhanced liquid condensation, resistance to ice formation, resistance to fogging, reduced drag in liquids, resistance to gas hydrate adhesion, provide antifouling properties, inhibition of corrosion, provide semi-permanent lubrication, present self-cleaning properties, and/or prevent microbial colonization which can be prepared without the limitations described above.
Therefore, it is an object of the invention to provide robust non-wetting surfaces that afford one or more of the following features: repellency to a variety of liquids, enhanced liquid condensation, resistance to ice formation, resistance to fogging, reduced drag in liquids, resistance to gas hydrate adhesion, provide antifouling properties, inhibition of corrosion, provide semi-permanent lubrication, present self-cleaning properties, and/or prevent microbial colonization which can be prepared without the limitations described above.